In the telecommunications industry and other applications, it is often necessary to test circuits having a very low impedance. Such tests are conventionally performed by computers which selectively activate test connections to the equipment by means of electronic switches. Unfortunately, the impedance of the switch itself becomes a significant factor in this type of test when the impedance to be tested is very low.
Two types of test circuitry are conventionally used for this purpose. In one type, computer-operated drivers actuate reed switches. These have a very low impedance (on the order of 2.OMEGA.) but have a deteriorating life in which their impedance gradually increases. In the other type, semiconductor switches are used to directly close the measuring circuit. These devices have unpredictable impedances varying in a range on the order of 60-200.OMEGA., and they are usually unable to handle much current. Specialized semiconductor switches can bring the switch impedance down to the order of less than 25.OMEGA., but this requires current levels on the order of 50A, which may be impractical.
Conventionally, these problems are solved by matching the semiconductor devices and using compensating circuits or bridges for nulling the effect of the switch impedance. Both of these solutions, however, are expensive and not entirely satisfactory.